Conditions of general $Z_{2}$ symmetry and TM$_{1,2}$ mixing for the minimal type-I seesaw mechanism in an arbitrary basis

Abstract

In this paper, using a formula for the minimal type-I seesaw mechanism by $LDL^{T}$ (or generalized Cholesky) decomposition, conditions of general $Z_{2}$-invariance for the neutrino mass matrix $m$ is obtained in an arbitrary basis. The conditions are found to be $(M_{22} a_{i}^{+} - M_{12} b_{i}^{+}) \, ( M_{22} a_{j}^{-} - M_{12} b_{j}^{-}) = - \det M \, b_{i}^{+} \, b_{j}^{-}$ for the $Z_{2}$-symmetric and -antisymmetric part of a Yukawa matrix $Y_{ij}^{\pm} \equiv (Y \pm T Y )_{ij} /2 \equiv (a_{j}^{\pm}, b_{j}^{\pm})$ and the right-handed neutrino mass matrix $M_{ij}$. In other words, the symmetric and antisymmetric part of $b_{i}$ must be proportional to those of the quantity $\tilde a_{i} \equiv a_{i} - {M_{12} \over M_{22}} b_{i}$. They are equivalent to the condition that $m$ is block diagonalized by eigenvectors of the generator $T$. These results are applied to three $Z_{2}$ symmetries, the $\mu-\tau$ symmetry, the TM$_{1}$ mixing, and the magic symmetry which predicts the TM$_{2}$ mixing. For the case of TM$_{1,2}$, the symmetry conditions become $ M_{22}^{2} \, \tilde {a}_{1}^{\rm TBM} \tilde a_{2}^{\rm TBM} = - \det M \, b_{1}^{\rm TBM} b_{2}^{\rm TBM}$ and $ M_{22}^{2} \, \tilde {a}_{1,2}^{\rm TBM} \tilde a_{3}^{\rm TBM} = - \det M \, b_{1,2}^{\rm TBM} b_{3}^{\rm TBM}$ with components $\tilde a_{i}^{\rm TBM}$ and $b_{i}^{\rm TBM}$ in the TBM basis $\mathbf{v}_{1,2,3}$. In particular, for the TM$_{2}$ mixing, the magic (anti-)symmetric Yukawa matrix with $S_{2} Y = \pm Y$ is phenomenologically excluded because it predicts $m_{2}=0$ or $m_{1}, m_{3} = 0$. In the case where Yukawa is not (anti-)symmetric, the mass singular values are displayed without a root sign.